The subject matter relates to a device for use in the estimation of deep tissue temperature (DTT) as an indication of the core body temperature of humans or animals. More particularly, the subject matter relates to constructions of zero-heat-flux DTT measurement devices with provision for thermal sensor calibration.
Deep tissue temperature measurement is the measurement of the temperature of organs that occupy cavities of human and animal bodies (core body temperature). DTT measurement is desirable for many reasons. For example, maintenance of core body temperature in a normothermic range during the perioperative cycle has been shown to reduce the incidence of surgical site infection; and so it is beneficial to monitor a patient's body core temperature before, during, and after surgery. Of course noninvasive measurement is highly desirable, for the safety and the comfort of a patient, and for the convenience of the clinician. Thus, it is most advantageous to obtain a noninvasive DTT measurement by way of a device placed on the skin.
Noninvasive measurement of DTT by means of a zero-heat-flux device was described by Fox and Solman in 1971 (Fox R H, Solman A J. A new technique for monitoring the deep body temperature in man from the intact skin surface. J. Physiol. January 1971: 212(2): pp 8-10). The Fox/Solman system, illustrated in FIG. 1, estimates core body temperature using a temperature measurement device 10 with a controlled heater of essentially planar construction that stops or blocks heat flow through a portion of the skin. Because the measurement depends on the absence of heat flux through the skin area where measurement takes place, the technique is referred to as a “zero heat flux” (ZHF) measurement. Togawa improved the Fox/Solman technique with a DTT measurement device structure that accounted for multidimensional heat flow in tissue. (Togawa T. Non-Invasive Deep Body Temperature Measurement. In: Rolfe P (ed) Non-Invasive Physiological Measurements. Vol. 1. 1979. Academic Press, London, pp. 261-277). The Togawa device, illustrated in FIG. 2, encloses Fox and Solman's ZHF design in a thick aluminum housing with a cylindrical annulus construction that reduces or eliminates radial heat flow from the center to the periphery of the device.
The Fox/Solman and Togawa devices utilize heat flux normal to the body to control the operation of a heater that blocks heat flow from the skin through a thermal resistance in order to achieve a desired ZHF condition. This results in a construction that stacks the heater, thermal resistance, and thermal sensors of a ZHF temperature measurement device, which can result in a substantial vertical profile. The thermal mass added by Togawa's cover improves the stability of the Fox/Solman design and makes the measurement of deep tissue temperature more accurate. In this regard, since the goal is zero heat flux through the device, the more thermal resistance the better. However, the additional thermal resistance adds mass and size, and also increases the time required to reach a stable temperature.
The size, mass, and cost of the Fox/Solman and Togawa devices do not promote disposability. Consequently, they must be sanitized after use, which exposes them to wear and tear and undetectable damage. The devices must also be stored for reuse. As a result, use of these devices raises the costs associated with zero-heat-flux DTT measurement and can pose a significant risk of cross contamination between patients. It is thus desirable to reduce the size and mass of a zero-heat-flux DTT measurement device, without compromising its performance, in order to promote disposability after a single use.
An inexpensive, disposable, zero-heat-flux DTT measurement device is described and claimed in the priority application and illustrated in FIGS. 3 and 4. The device is constituted of a flexible substrate and an electrical circuit disposed on a surface of the flexible substrate. The electrical circuit includes an essentially planar heater which is defined by an electrically conductive copper trace and which surrounds an unheated zone of the surface, a first thermal sensor disposed in the zone, a second thermal sensor disposed outside of the heater trace, a plurality of electrical pads disposed outside of the heater trace, and a plurality of conductive traces connecting the first and second thermal sensors and the heater trace with the plurality of electrical pads. Sections of the flexible substrate are folded together to place the first and second thermal sensors in proximity to each other. A layer of insulation disposed between the sections separates the first and second thermal sensors. The device is oriented for operation so as to position the heater and the first thermal sensor on one side of the layer of insulation and the second thermal sensor on the other and in close proximity to an area of skin where a measurement is to be taken. As seen in FIG. 4, the layout of the electrical circuit on a surface of the flexible substrate provides a low-profile, zero-heat-flux DTT measurement device that is essentially planar, even when the sections are folded together.
Design and manufacturing choices made with respect to a zero-heat-flux DTT measurement device can influence the operation of the device. One such design choice relates to the thermal sensors used in the detection of the zero-heat-flux condition. Given the importance of core body temperature, it is very desirable that the thermal sensors produce accurate temperature data in order to enable reliable detection of the zero-heat-flux condition and accurate estimation of core body temperature. The tradeoff is between accuracy and cost of the thermal sensor. A number of thermal sensor devices are candidates for use in zero-heat-flux DTT measurement. Such devices include PN junctions, thermocouples, resistive temperature devices, and thermistors, for example. Thermistors are a good choice for reasons of small size, handling convenience, ease of use, and reliability in the temperature range of interest. Their relatively low cost makes them desirable candidates for single-use, disposable temperature measurement devices.
The magnitude of a thermistor's resistance changes in response to a change of the temperature of the thermistor. Thus, to determine the magnitude of the temperature, the thermistor's resistance is measured and converted to a temperature value using a known relationship. However, batch-to-batch manufacturing variances can yield a large range variance in thermistor resistance. For example, low-cost thermistors can exhibit a range of ±5% in resistance values from device to device at a given temperature, which yields a range of ±2.5° C. in temperature. Such a large range in variance can compromise the accuracy and reliability of zero-heat-flux temperature measurement. Thus, while it is desirable to use such thermistors in order to limit the cost of parts and labor in manufacturing zero-heat-flux DTT measurement devices, it is important to reduce, if not remove, the effects of resistance variance on device operation.
The range of thermistor resistance variance can be neutralized by calibration of thermistor resistance using known methods, such as the Steinhart-Hart equation, which require knowledge of coefficients derived from values of thermistor resistance measured at fixed temperatures. When a thermistor is operated, the coefficients are used in known formulas to correct or adjust the magnitude of its indicated resistance. Such correction is called calibration.
Preferably, once determined, the coefficients are stored in a memory device so as to be available for use when the thermistor is operated. For example, as described in Japanese patent publication 2002-202205, a deep temperature measuring device includes a temperature probe constructed for zero-heat-flux measurement and a cable projecting from the probe. One end of the cable terminates on the probe, and the opposite end in a connector. Signal wires run in the cable between the probe and the connector. A read-only memory (ROM) is mounted in the connector casing, away from the probe. Information stored in the ROM includes probe classification and thermistor coefficients. Since the thermistor coefficients are unique to the thermistors on the probe, the ROM must be permanently associated with the probe, and so the cable is permanently fixed to the probe. The connector detachably plugs into a temperature measurement system. At start-up, the system reads the classification and coefficient information from the ROM. The system uses the coefficient information to calibrate thermistor readings obtained from the probe, thereby to reduce or remove the effects of resistance variation from the zero-heat-flux process.
The cable of the deep temperature measuring device with its permanent connector results in a complex construction that is costly to manufacture, difficult to store, and awkward to handle. A full complement of probes for a temperature measuring system has as many cables as probes. The probes are reusable, and so the problems described above in connection with the Fox/Solman and Togawa devices are compounded by the presence of the cables.